Synthesis, Radiolabeling, and Evaluation for β-Amyloid Plaque Imaging

Sep 14, 2006 - Department of Nuclear Medicine, Samsung Medical Center, Sungkyunkwan UniVersity School of Medicine, 50 Ilwon-dong, Kangnam-ku,...
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J. Med. Chem. 2006, 49, 6111-6119

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Curcumin and Dehydrozingerone Derivatives: Synthesis, Radiolabeling, and Evaluation for β-Amyloid Plaque Imaging† Eun Kyoung Ryu, Yearn Seong Choe,* Kyung-Han Lee, Yong Choi, and Byung-Tae Kim Department of Nuclear Medicine, Samsung Medical Center, Sungkyunkwan UniVersity School of Medicine, 50 Ilwon-dong, Kangnam-ku, Seoul 135-710, Korea ReceiVed June 15, 2006

Alzheimer’s disease (AD) is pathologically characterized by the accumulation of amyloid plaques and neurofibrillary tangles in the brain, and thus, the in vivo imaging of plaques and tangles would be beneficial for the early diagnosis of AD. It has been suggested that 5-hydroxy-1,7-bis(4-hydroxy-3-methoxyphenyl)1,4,6-heptatrien-3-one (curcumin) may be responsible for low age-adjusted prevalence of AD in India. In the present study, eight novel derivatives of curcumin and 4-(4-hydroxy-3-methoxyphenyl)-3-buten-2-one (dehydrozingerone) were synthesized and their binding affinities for β-amyloid (Aβ) aggregates were measured. Of these ligands, fluoropropyl-substituted curcumin (8) showed the highest binding affinity (Ki ) 0.07 nM), and therefore, 8 was radiolabeled and evaluated as a potential probe for Aβ plaque imaging. Partition coefficient measurement and biodistribution in normal mice demonstrated that [18F]8 has a suitable lipophilicity and reasonable initial brain uptake. Metabolism studies also indicated that [18F]8 is metabolically stable in the brain. These results suggest that [18F]8 is a suitable radioligand for Aβ plaque imaging. Introduction (AD)a

Alzheimer’s disease is a progressive neurodegenerative disorder and a leading cause of dementia. AD is characterized by abundant senile plaques composed of Aβ peptides and numerous neurofibrillary tangles formed from filaments of highly phosphorylated tau proteins, which are invariably observed in the postmortem brain tissues of AD patients.1-3 Therefore, the monitoring of Aβ plaques and neurofibrillary tangles may be beneficial for the diagnosis, staging, and treatment of AD. Many specific ligands have been developed for the imaging of Aβ plaques. These are based on fluorescent dyes that are used to stain Aβ plaques and neurofibrillary tangles in vitro and include highly conjugated symmetric compounds, such as Congo red (CR) and chrysamine-G (CG), and a smaller molecule, such as thioflavin-T (Figure 1).4 However, radioiodine-labeled CR has low brain uptake, probably because of the hydrophilic nature of its sulfonate groups.5 The more lipophilic CG has been labeled with 125I or 99mTc-monoamine-monoamide bisthiol (MAMA), but studies showed low brain uptake in rodents.6-8 Lansbury and co-workers synthesized the positively charged 99mTc(bipyridyl)(t-butylisonitrile)4 derivatives of CR and CG, and found that they had Kd values for Aβ(1-40) comparable to those of CR for insulin amyloid fibrils and of CG for Aβ(10-43).9 This group further prepared neutral Re(dO)-MAMA-CR and CG complexes and a Tc(dO)MAMA-CR complex. The Re complexes showed affinities for Aβ(40) comparable to those of CR and CG and stained amyloid plaques in AD brain sections.10 However, the complexes may exceed the physicochemical and molecular weight limitations (700 Daltons) imposed by the blood-brain barrier (BBB).4,11,12 In other studies, CG was further modified to increase brain * Corresponding author. Tel.: +82-2-3410-2623. Fax: +82-2-34102639. E-mail: [email protected]. † Presented in part at the International Chemical Congress of Pacific Basin Societies, Honolulu, HI, December 15-20, 2005. a Abbreviations: AD, Alzheimer’s disease; Aβ, β-amyloid; BBB, bloodbrain barrier; PET, positron emission tomography; SPECT, single photon emission computed tomography.

uptake; diazo groups and a biphenyl group were replaced by vinyl groups and a phenyl group, respectively, in 1,4-bis(3hydroxycarbonyl-4-hydroxyphenylethenyl)benzene (X-34, Ki ) 17.8 nM),4,13 1-bromo-2,5-bis(3-hydroxycarbonyl-4-hydroxy)styrylbenzene (BSB, Ki ) 400 nM),14 1-iodo-2,5-bis(3-hydroxycarbonyl-4-hydroxy)styrylbenzene (ISB, Kd ) 0.08 nM), and 1-iodo-2,5-bis(3-hydroxycarbonyl-4-methoxy)styrylbenzene (IMSB, Ki ) 0.17 nM; Figure 1).15 Radioiodine-labeled ISB and IMSB showed lower initial brain uptakes in normal mice (0.27% ID/organ and 0.14% ID/organ at 5 min after injection) than radioiodinated thioflavin-T derivatives (0.6-3.5% ID/organ at 2 min), despite their potent binding affinities for Aβ aggregates.15 This may be because that the carboxylic acids of CG derivatives would be mostly ionized in the physiological pH range, which would reduce their abilities to cross the BBB. A neutral and lipophilic fluorescent dye, 2-(1-{6-[dimethylamino]-2-naphthyl}ethylidene)malononitrile (DDNP) was also modified for 18F labeling. A fluoromethyl derivative of DDNP (FDDNP) was found to have a Kd of 0.12 nM for high binding sites on Aβ aggregates and a Kd of 1.86 nM for low binding sites. [18F]FDDNP was found to easily traverse the BBB due to its high lipophilicity (log P ) 3.92). This radioligand was found to label both senile plaques and neurofibrillary tangles in the brains of AD patients with positron emission tomography (PET).16,17 A charged molecule, thioflavin-T, was structurally modified to improve its brain permeability;4,18,19 the neutral benzothiazole derivatives obtained include 2-[4′-(dimethylamino)phenyl]-6iodobenzothiazole (TZDM, Ki ) 1.9 nM),15 2-(4′-dimethylaminophenyl)-6-iodobenzoxazole (IBOX, Ki ) 0.8 nM),20 6-iodo2-(4′-(dimethylamino)-)phenyl-imidazo[1,2-a]pyridine (IMPY, Ki ) 15 nM),21 and 2-(4′-(methylamino)phenyl-6-hydroxybenzothiazole (6-OH-BTA-1, Ki ) 4.3 nM; Figure 1),22 However, although [125I]TZDM and [125I]IBOX labeled Aβ in postmortem AD brain sections, they showed slow washout from normal mouse brain.15,20 Radioiodine-labeled benzofuran derivatives also showed slow radioactivity washout from normal mouse brain, indicating a high level of nonspecific binding in vivo.23 However, [125I]IMPY showed good initial brain uptake (2.88%

10.1021/jm0607193 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/14/2006

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Figure 1. Chemical structures of CR, CG, styrylbenzene derivatives, FDDNP, thioflavin-T derivatives, and stilbene derivative.

ID/organ at 2 min) and rapid washout from normal mouse brain,21 and labeled regions containing Aβ in transgenic PSAPP mouse brain sections and also in postmortem AD brain sections.24,25 [11C]6-OH-BTA-1 (PIB), a benzothiazole-aniline derivative, was also found to have high initial uptake and rapid clearance from normal mouse and baboon brains,22 and human trials with this radioligand showed high uptake in brains of AD patients and significant retention in frontal and temporoparietal cortex, the anterior and posterior cingulated cortex, and the striatum, which is consistent with the known pattern of Aβ plaque distribution in the AD brain.4,26,27 Stilbene derivatives were also found to have good binding affinities for Aβ aggregates in vitro (Ki ) 1.2-151 nM). In particular, 11C-labeled 4-N-methylamino-4′-hydroxystilbene (SB13 (Figure 1), Ki ) 6.0 nM) displayed moderate lipophilicity (log P ) 2.36), high initial brain uptake in the normal rat cortex (1.51% ID/g at 2 min), and rapid washout (0.42% ID/g at 30 min). This radioligand also showed specific labeling of Aβ plaques in brain sections from transgenic (TgCRDN8) mice.28 5-Hydroxy-1,7-bis(4-hydroxy-3-methoxyphenyl)-1,4,6-heptatrien-3-one (curcumin) has been extensively studied for its anticancer, antioxidant, and anti-inflammatory activities.29-31 Moreover, it has also been reported that the age-adjusted prevalence of AD patients in India is 4.4-fold less than that in the U.S.A.,32 and it was suggested that this may be explained by the fact that curcumin, a major ingredient of the curry spice turmeric, is used as a food preservative and herbal medicine in India.33 Moreover, it has a favorable toxicity profile in mice and was found to cause no adverse effects in cancer patients during clinical studies.29,33 In addition, curcumin has been reported to have favorable brain permeability, and satisfactory Aβ plaque binding properties have been predicted from the fluorescence staining of Aβ plaques in brain sections from APPsw transgenic mice administered curcumin by injection or in diet.34 4-(4-Hydroxy-3-methoxyphenyl)-3-buten-2-one (dehydrozingerone), a partial structure of curcumin, is also known to have antioxidant properties.35

Scheme 1a

a

Reagents and conditions: (a) acetone, 2.5 M NaOH, rt, 24 h, 54%.

In the present study, curcumin and dehydrozingerone derivatives were synthesized and evaluated in vitro and in vivo as Aβ plaque imaging probes for PET or single photon emission computed tomography (SPECT). Results and Discussion Chemistry. The syntheses of dehydrozingerone and curcumin derivatives are shown in Schemes 1-4. Dehydrozingerone derivatives, 1-3, were prepared by aldol condensations of the vanillin derivatives and acetone in the presence of 2.5 M NaOH (Schemes 1 and 2).36 Recrystallization from ethanol and water yielded orange-colored products. Fluoroalkyl derivatives of dehydrozingerone, 2 and 3, were prepared as in Scheme 2. Compounds 9 and 10 were prepared in high yields from the O-alkylation of vanillin with the corresponding dibromoalkanes in the presence of K2CO3. The bromo groups of 9 and 10 were converted into tosyl groups, as in 11 and 12, using silver p-toluenesulfonate under reflux overnight, and these were further substituted with fluorine atoms using n-Bu4NF to give 13 and 14. Aldol condensation of the resulting aldehyde and acetone, followed by dehydration, afforded the dehydrozingerone derivatives 2 and 3. Aldol condensation of acetylacetone-boron complex and vanillin derivatives is a key step in acetylation of the dehydrozingerone in ligands 4-6.37 Keto-enol compounds (4, 5, 6, 15, and 16) were synthesized by reacting the corresponding aldehydes with 2 equiv of acetylacetone-boron complex. Recrystallization from ethanol and water gave products that were visualized by fluorescence and UV illumination. Curcumin derivatives, 7, 8, and 17 were synthesized by coupling keto-enol compounds and the corresponding vanillin

Curcumin and Dehydrozingerone DerViatiVes

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Scheme 2a

a Reagents and conditions: (a) Br(CH ) Br, K CO , DMF, 80 °C, 1 h, 65-99%; (b) AgOTs, CH CN, reflux, overnight, 66-85%; (c) n-Bu NF, THF, 2 n 2 3 3 4 reflux, 4 h, 74-91%; (d) acetone, 2.5 M NaOH, rt, 24 h, 68-83%; (e) 2,4-pentanedione, B2O3, (n-BuO)3B, ethyl acetate, 80 °C, 30 min, n-BuNH2, 100 °C, 30 min, 1 N HCl, 50 °C, 30 min, 48-53%.

Scheme 3a

a Reagents and conditions: (a) 2,4-pentanedione, B O , (n-BuO) B, ethyl acetate, 80 °C, 30 min, n-BuNH , 100 °C; 30 min, 1 N HCl, 50 °C, 30 min, 2 3 3 2 59-85%; (b) vanillin, B2O3, (n-BuO)3B, ethyl acetate, 80 °C, piperidine, 80 °C, 30 min, 0.4 N HCl, 50 °C, 30 min, 50-58%.

Scheme 4a

a Reagents and conditions: (a) 15, B O , (n-BuO) B, ethyl acetate, 80 °C, piperidine, 80 °C, 30 min, 0.4 N HCl, 50 °C, 30 min, 37.6%; (b) AgOTs, 2 3 3 CH3CN, reflux, overnight, 62%; (c) n-Bu4NF, THF, reflux, 4 h, 23%; (d) 15, B2O3, (n-BuO)3B, ethyl acetate, 80 °C, piperidine, 80 °C, 30 min, 0.4 N HCl, 50 °C, 30 min, 54%.

derivative-boron complexes in the presence of piperidine, which was found to give better yields than when n-butylamine was used (Schemes 3 and 4).37 Direct iodination of curcumin to produce 7 yielded a side product that probably resulted from iodination at the C4 position. To prepare the radioligands ([125I]6 and [125I]7) with high specific activity, bromo compounds were used instead of iodo compounds to synthesize the precursors shown in Schemes 5 and 6. Bromo groups were converted to the tributylstannyl precursors, 20 and 21, using bis(tributyltin) in the presence of Pd(PPh3)4. The fluoropropyl derivative of curcumin was synthesized using two different methods, as shown in Scheme 4. The tosylate precursor 19 was prepared by coupling 10 and the 15-boron complex, and this was followed by substituting the bromo group with a tosyl group. However, the subsequent substitution with n-Bu4NF to give ligand 8 was only achieved in a yield of 23%. As an alternative pathway, the hydroxyl group of 18 was protected as an acetyl ester with acetic anhydride in pyridine in high yield, and this was followed by substitution of the bromo group with a tosyl group. However, fluorination of the tosylate

Scheme 5a

a Reagents and conditions: (a) Pd(PPh ) , (SnBu ) , toluene, 110 °C, 3 4 3 2 overnight, 20%; (b) Na125I, 1 N HCl, 3% H2O2, ethanol, rt, 10 min, 1014%.

precursor also had a low yield (13%), and therefore, 14 was coupled with 15 to prepare the product 8 (32%). Radiolabeling. [125I]6 and [125I]7 were prepared by radioiododestannylation of the corresponding tributylstannyl precur-

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Scheme 6a

a

Reagents and conditions: (a) Pd(PPh3)4, (SnBu3)2, toluene, 110 °C, overnight, 19%; (b) Na125I, 1 N HCl, 3% H2O2, rt, 10 min, 35-40%.

Scheme 7a

a Reagents and conditions: (a) n-Bu N18F, CH CN, 105 °C, 10 min, >85%; (b) 15, B O , (n-BuO) B, piperidine, ethyl acetate, 105 °C, 15 min, 0.4 N 4 3 2 3 3 HCl, 90 °C, 5 min, 16-25%; (c) n-Bu4N18F, THF, 95 °C, 20 min, 0-17%.

sors, 20 and 21, respectively, using Na[125I]I in the presence of 3% H2O2 at room temperature for 10 min (Schemes 5 and 6). The reaction was quenched with saturated NaHSO3, and the mixture was purified by HPLC. Decay-corrected radiochemical yields of [125I]6 and [125I]7 were 10-14% and 35-40% and specific activities were 72.6 GBq/µmol and 87.3 GBq/µmol, respectively. Radioligand [18F]8 was synthesized as shown in Scheme 7. When precursor 19 was used for 18F-labeling, [18F]8 was synthesized in variable yields (0-17%). As an alternative pathway, nucleophilic [18F]fluorination of acetyl protected tosylate precursor also gave the product in low yield (13%) and with low effective specific activity. Therefore, [18F]14, which was prepared from 12 and n-Bu4N[18F]F (>85%), was used for the preparation of [18F]8. The subsequent aldol condensation with acetylacetone-boron complex, followed by HPLC purification, gave the product in overall decay-corrected radiochemical yield of 16-25% and with an effective specific activity of 37.6 GBq/µmol. Binding Assays. In vitro binding assays using Aβ(1-40) aggregates were performed as described in the literature,15,18 and radiolabeled standards, [125I]IMSB and [125I]IMPY, were prepared as previously described.15,21 Their Kd values for binding to Aβ(1-40) aggregates were similar to those reported, that is, 0.18 nM versus 0.13 nM (reported) for [125I]IMSB and 17.3 nM versus 5.4 nM (reported value obtained using the gray matter homogenates of AD brains) for [125I]IMPY.15,25 The binding affinities of ligands described here were determined using Aβ aggregates and the radiolabeled standards, [125I]IMSB or [125I]IMPY, depending on the structures of the ligands tested, and nonspecific binding was measured in the presence of CG or thioflavin-T (Table 1). [125I]IMSB and CG were used for curcumin derivatives, but [125I]IMPY and thioflavin-T were used for dehydrozingerone derivatives. Dehydrozingerone derivatives, including 1-6, showed lower affinities for Aβ aggregates than curcumin derivatives. In particular, 2-fluoroalkyl ligands, 2-5, exhibited Ki values higher than 200 nM for the Aβ aggregates. On the other hand, the iodine-substituted ligands, 1 and 6, showed reasonable Ki values (36.6 nM and 24.9 nM, respectively) for Aβ aggregates. Dehydrozingerone was found to have similar affinities for Aβ aggregates upon using [125I]IMPY or [125I]IMSB (55.9 nM vs 54.2 nM, respectively) as the radiolabeled standards, and ligand

Table 1. Ki (nM) of Ligands for Aβ(1-40) Aggregates ligand

[125I]IMPY

[125I]IMSB

dehydrozingerone 1 2 3 4 5 6 CG curcumin 7 8

55.9 ( 4.0 36.6 ( 1.2 232.7 ( 16 839.2 958.1 ( 93 316.3 ( 69 24.9 ( 0.9

54.24 19.97 ( 2.7

0.16 ( 0.04 0.20 ( 0.06 9.37 ( 1.0 0.07 ( 0.01

1 competed better with [125I]IMSB binding than [125I]IMPY binding to Aβ aggregates (19.97 nM vs 36.6 nM). CG showed a similar Ki value (0.16 nM) to that reported (0.14 nM) for binding to Aβ(1-40) aggregates.15 Curcumin exhibited potent binding affinity for Aβ(1-40) aggregates (Ki ) 0.20 nM), but the introduction of an iodine into its phenyl ring (7) significantly decreased its binding affinity by 47-fold, whereas O-fluoropropylation of its hydroxyl group (8) increased binding affinity 2.9-fold, giving a Ki value of 0.07 nM. These results demonstrate that a highly conjugated dimeric structure is required for high Aβ aggregate binding affinity. Partition Coefficient Measurement. The partition coefficients of some ligands were measured using radiolabeled forms, that is, [125I]6, [125I]7, and [18F]8 had log Po/w values of 1.35, 0.94, and 1.84, respectively, predicting a favorable brain permeability for all three. Of these radioligands, [18F]8 is most preferable for in vivo evaluations because of its high binding affinity and suitable lipophilicity. Metabolism Studies. Radioligand [18F]8 was analyzed using mouse brain and blood and remained intact in the brain during the time of the study (60 min), whereas an unidentified polar metabolite appeared in blood samples within 2 min after injection and [18F]8 was completely converted into the metabolite at 60 min (Figure 2). This polar radioactive metabolite did not seem to cross the BBB because of its high polarity. These results demonstrate that [18F]8 is metabolically stable in the mouse brain after being taken up. Biodistribution. Radioligands ([125I]6 and [18F]8) were evaluated in normal mice to assess their ability to cross the BBB. Radioligand [125I]6 showed a similar initial brain uptake (0.69% ID/g) to [18F]8, but lower radioactivity accumulation in liver,

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Journal of Medicinal Chemistry, 2006, Vol. 49, No. 20 6115 Table 3. Biodistribution of [18F]8 (A) and of [18F]8 Co-Injected with Piperine (B) in Normal Mice (A) organ

2 min

30 min

60 min

blood heart lung liver spleen kidney muscle bone brain

7.34 ( 1.29 4.05 ( 0.40 30.91 ( 4.43 38.99 ( 2.80 16.32 ( 1.99 8.64 ( 0.35 0.89 ( 0.12 3.23 ( 0.29 0.52 ( 0.08

1.24 ( 0.01 0.57 ( 0.08 7.23 ( 0.34 41.06 ( 5.41 42.94 ( 1.19 2.71 ( 1.11 0.28 ( 0.16 2.68 ( 0.11 0.11 ( 0.03

0.61 ( 0.45 0.64 ( 0.08 10.55 ( 1.73 37.06 ( 6.24 36.05 ( 9.45 2.07 ( 0.58 0.27 ( 0.09 3.43 ( 0.48 0.11 ( 0.02

(B)

Figure 2. Metabolism of [18F]8 in mouse brain and blood. Data are presented as % parent radioligand in brain (b) and in blood (9) as determined by radio-TLC as a function of time (2, 30, and 60 min). Table 2. Biodistribution of [125I]6 in Normal Mice organ

2 min

30 min

60 min

120 min

blood thyroid heart lung liver spleen kidney muscle brain

17.92 ( 7.65 4.60 ( 0.70 5.23 ( 0.87 14.78 ( 9.86 18.28 ( 0.60 4.61 ( 1.31 11.82 ( 1.37 1.35 ( 0.13 0.69 ( 0.08

2.05 ( 0.47 10.25 ( 1.44 0.72 ( 0.13 1.30 ( 0.35 4.64 ( 1.23 1.93 ( 0.46 9.63 ( 1.17 0.54 ( 0.14 0.11 ( 0.05

0.87 ( 0.05 23.18 ( 4.05 0.31 ( 0.02 0.57 ( 0.06 2.85 ( 0.51 1.81 ( 0.44 2.34 ( 0.34 0.26 ( 0.07 0.05 ( 0.01

0.64 ( 0.09 27.63 ( 13.0 0.20 ( 0.05 0.57 ( 0.20 2.53 ( 0.42 1.37 ( 0.40 1.03 ( 0.10 0.13 ( 0.04 0.02 ( 0.01

spleen, and lungs (Table 2). The high thyroid uptake of [125I]6 indicated that this radioligand was deiodinated in vivo (4.6% ID/g at 2 min, 10.3% ID/g at 30 min, and 23.2% ID/g at 60 min). Biodistribution studies on [18F]8 demonstrated that high and persistent radioactivity accumulation occurred in the liver and spleen and that its lung uptake decreased with time. Brain uptake of [18F]8 was 0.52% ID/g at 2 min postinjection, and its radioactivity was rapidly washed out from the brain at 30 min (0.11% ID/g) (Table 3A). In addition, [18F]8 did not appear to undergo metabolic defluorination due to a constant level of bone uptake with time (2.7-3.4% ID/g). However, despite its suitable lipophilicity and reasonable molecular size (mol wt 428.45), its initial brain uptake was relatively low, which may be explained by its rapid metabolism in the liver and in the intestinal wall, like curcumin.38 Piperine, a major component of black pepper, is a known inhibitor of hepatic and intestinal glucuronidation and is also shown to increase the bioavailability of curcumin. This effect of piperine on the pharmacokinetics of curcumin has been shown to be much greater in humans than in rats; in humans, curcumin bioavailability was increased by 2000% at 45 min after co-administering curcumin orally with piperine, whereas in rats, curcumin bioavailability was increased by only 154% between 1 and 2 h after co-administration.38 Therefore, the effect of piperine on the uptake of [18F]8 was assessed after co-injecting [18F]8 with piperine. As shown in Table 3B, initial brain uptake (0.77% ID/g at 2 min) was increased by 48% (p value < 0.05) relative to that without piperine, although other organ uptakes were similar to those without piperine (Table 3A,B). However, the effect of piperine was limited, possibly due to its poor solubility in 10% ethanolsaline. Therefore, the initial brain uptake of [18F]8 would be

organ

2 min

30 min

60 min

120 min

blood heart lung liver spleen kidney muscle brain

11.18 ( 3.41 5.24 ( 1.10 34.26 ( 6.81 35.39 ( 3.93 28.68 ( 7.26 8.69 ( 1.04 1.12 ( 0.18 0.77 ( 0.10

0.90 ( 0.05 0.70 ( 0.06 13.61 ( 1.33 43.13 ( 4.97 36.25 ( 8.36 2.81 ( 0.73 0.36 ( 0.07 0.14 ( 0.02

0.82 ( 0.11 0.57 ( 0.03 7.88 ( 0.62 37.02 ( 4.77 41.07 ( 1.66 1.70 ( 0.41 0.28 ( 0.03 0.09 ( 0.02

0.75 ( 0.16 0.38 ( 0.08 3.48 ( 1.26 18.53 ( 7.85 21.49 ( 15.0 0.99 ( 0.22 0.20 ( 0.12 0.07 ( 0.02

significantly increased if the solubility of piperine can be enhanced in an injection solution. An initial brain uptake higher than 0.5% ID/organ at 2 min after injection is preferred for Aβ plaque imaging probes, and this initial uptake should remain at less than 30% at 30 min in normal mouse brain because of the absence of Aβ plaques.21,24 Although the initial brain uptakes of [125I]6 and [18F]8 fall slightly short of these criteria (0.24-0.34% ID/organ at 2 min), these radioligands showed rapid washout at 30 min. Thus, taken together with other in vitro and in vivo results, [125I]6 may be unsuitable for Aβ plaque imaging because of its severe in vivo deiodination. In contrast, [18F]8 has promise as an Aβ plaque imaging probe because it is metabolically stable in the brain and, therefore, it would strongly bind to Aβ plaques due to its excellent in vitro binding affinity (Ki ) 0.07 nM) once taken up by the brain. Moreover, the fluorescence of curcumin derivatives aids locating Aβ plaques in vitro and, thus, offers an additional advantage. Conclusion Eight novel curcumin and dehydrozingerone derivatives were synthesized and evaluated in vitro and in vivo. The curcumin derivatives showed higher binding affinities for Aβ(1-40) aggregates than dehydrozingerone derivatives, with 8 being the most potent ligand. Moreover, [18F]8 showed suitable lipophilicity, reasonable initial brain uptake, and metabolic stability in the normal mouse brain. These results suggest that [18F]8 is a promising candidate for Aβ plaque imaging. Experimental Section Chemicals were purchased from Sigma-Aldrich (St. Louis, MO) and CG from Merck Biosciences (San Diego, CA). Aβ(1-40) peptide was obtained from Biosource, Inc. (Camarillo, CA), and Na[125I]I from PerkinElmer Life and Analytical Sciences (Boston, MA). Dehydrozingerone and curcumin were synthesized as described in the literature.36,37 1H NMR spectra were obtained using a Varian UnityInoVa 500NB (500 MHz) spectrometer (Palo Alto, CA), and chemical shifts (δ) were reported in ppm downfield from tetramethylsilane. Electron impact (EI) and fast atom bombardment (FAB) mass spectra were obtained using a JMS-700 Mstation instrument (JEOL, Ltd., Tokyo, Japan). Elemental analysis was performed on a Flash EA 1112 (ThermoQuest CE Instruments, Milan, Italy). Purification of radioligand was carried out on a HPLC

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(Thermo Separation Products System, Fremont, CA) equipped with a semipreparative column (YMC-Pack C18, 5 µ, 10 × 250 mm), and eluant was simultaneously monitored using a UV detector (254 nm) and a NaI(T1) radioactivity detector. Radio-thin-layer chromatography (TLC) was performed on Merck F254 silica plates and analyzed using a Bioscan scanner (Washington DC, U.S.A.). [18F]Fluoride was produced using the 18O(p,n)18F reaction on a PETtrace cyclotron (GE Healthcare, Uppsala, Sweden). Radioactivity was measured in a dose calibrator (Biodex Medical Systems, Shirley, NY), and tissue radioactivity was measured in a Wallac automated gamma counter (Boston, MA). All animal experiments complied with the rules of the Samsung Medical Center Laboratory Animal Care, which are based on NIH guidelines. 4-(2-Bromoethoxy)- and 4-(3-Bromopropoxy)-3-methoxybenzaldehyde (9 and 10). Vanillin (0.5 g, 3.29 mmol) and K2CO3 (0.64 g, 4.6 mmol) were added to 1,2-dibromoethane or 1,3dibromopropane (9.79 mmol) in DMF (20 mL). The reaction mixture was then stirred at 80 °C for 1 h, cooled to 0 °C on an ice bath, quenched with 1 N HCl, and extracted with ethyl acetate. The organic layer was washed with water and dried over Na2SO4. Flash column chromatography (2:1 hexanes-ethyl acetate) afforded 9 (550 mg, 65%) or 10 (890 mg, 99%) as white solids. 4-(2-Bromoethoxy)-3-methoxybenzaldehyde (9): mp 71.472.1 °C; 1H NMR (CDCl3) δ 3.71 (t, J ) 7.0 Hz, 2H), 3.96 (d, J ) 5.0 Hz, 3H), 4.43 (t, J ) 7.0 Hz, 2H), 7.00 (d, J ) 8.5 Hz, 1H), 7.46 (dd, J ) 5.8, 2.0 Hz, 2H), 9.89 (s, 1H); MS (EI) m/z 260 (M+, 81Br), 258 (M+, 79Br); HRMS calcd for C10H1181BrO3, 259.9872; found, 259.9875. 4-(3-Bromopropoxy)-3-methoxybenzaldehyde (10): mp 58.459.9 °C; 1H NMR (CDCl3) δ 2.39-2.44 (m, 2H), 3.64 (t, J ) 6.5 Hz, 2H), 3.92 (s, 3H), 4.25 (t, J ) 6.0 Hz, 2H), 7.01 (d, J ) 8.0 Hz, 1H), 7.42-7.46 (m, 2H), 9.86 (s, 1H); MS (EI) m/z 274 (M+, 81Br), 272 (M+, 79Br); HRMS calcd for C H 81BrO , 274.0029; 11 13 3 found, 274.0006. Anal. (C11H13BrO3) C, H. 4-(2-Tosyloxyethoxy)- and 4-(3-Tosyloxypropoxy)-3-methoxybenzaldehyde (11 and 12) and 5-Hydroxy-7-(4-hydroxy-3methoxyphenyl)-1-[3-methoxy-4-(3-tosyloxypropoxy)phenyl]1,4,6-heptatrien-3-one (19). To 9, 10, or 18 (2.93 mmol) in CH3CN (15 mL) was added silver p-toluenesulfonate (2 g, 7.17 mmol), and the reaction mixtures were stirred under reflux overnight. After removing solvent in vacuo, the resulting residues were extracted with CH2Cl2, washed with water, and dried over Na2SO4. Flash column chromatography (3:1 hexanes-ethyl acetate) gave 11 (680 mg, 66%), 12 (910 mg, 85%), or 19 (1.1 g, 62%) as white solids. 4-(2-Tosyloxyethoxy)-3-methoxybenzaldehyde (11): mp 105.5107.2 °C; 1H NMR (CDCl3) δ 2.45 (s, 3H), 3.92 (s, 3H), 4.324.34 (m, 2H), 4.43-4.45 (m, 2H), 6.93 (d, J ) 8.0 Hz, 1H), 7.34 (d, J ) 8.0 Hz, 2H), 7.42-7.43 (m, 2H), 7.83 (d, J ) 8.5 Hz, 2H), 9.87 (s, 1H); MS (EI) m/z 350 (M+); HRMS calcd for C17H18O6S, 350.0824; found, 350.0825. Anal. (C17H18O6S) C, H. 4-(3-Tosyloxypropoxy)-3-methoxybenzaldehyde (12): mp 97.698.2 °C; 1H NMR (CDCl3) δ 2.19-2.24 (m, 4H), 2.37 (s, 3H), 3.87 (s, 3H), 4.10 (t, J ) 6.0 Hz, 2H), 4.29 (t, J ) 6.0 Hz, 2H), 6.88 (d, J ) 8.0 Hz, 1H), 7.25 (d, J ) 5.0 Hz, 2H), 7.38-7.43 (m, 2H), 7.75 (dd, J ) 3.3, 2.0 Hz, 2H), 9.86 (s, 1H); MS (FAB) m/z 387 (M+ + Na); HRMS calcd for C18H20O6SNa, 387.0878; found, 387.0872. Anal. (C18H20O6S) C, H. 5-Hydroxy-7-(4-hydroxy-3-methoxyphenyl)-1-[3-methoxy-4(3-tosyloxypropoxy)phenyl]-1,4,6-heptatrien-3-one (19): 1H NMR (CDCl3) δ 1.21-1.27 (m, 2H), 2.38 (s, 3H), 3.85 (s, 3H), 3.95 (s, 3H), 4.00-4.05 (m, 2H), 4.27-4.32 (m, 2H), 5.82 (s, 1H), 5.86 (s, 1H), 6.49 (dd, J ) 16.0, 5.5 Hz, 2H), 6.77 (d, J ) 13.5 Hz, 1H), 6.94 (d, J ) 8.0 Hz, 1H), 7.02-7.13 (m, 4H), 7.40 (d, J ) 9.0 Hz, 2H), 7.59 (d, J ) 13.0 Hz, 2H), 7.76 (d, J ) 3.5 Hz, 2H); MS (EI) m/z 580 (M+); HRMS calcd for C31H32O9S, 580.1767; found, 580.1766. 4-(2-Fluoroethoxy)- and 4-(3-Fluoropropoxy)-3-methoxybenzaldehyde (13 and 14). Compound 11 or 12 (0.96 mmol) was dissolved in THF (10 mL), and to this solution was added n-Bu4NF (1 M in THF, 6 mL, 6 mmol). Reaction mixtures were refluxed for 4 h. After removing solvent in vacuo, products were purified

Ryu et al.

by flash column chromatography (3:1 hexanes-ethyl acetate) to afford 13 (140 mg, 74%), a white solid, or 14 (184.6 mg, 91%), a colorless oil. 4-(2-Fluoroethoxy)-3-methoxybenzaldehyde (13): mp 82.184.2 °C; 1H NMR (CDCl3) δ 3.96 (s, 3H), 4.35-4.37 (m, 1H), 4.40-4.42 (m, 1H), 4.79-4.81 (m, 1H), 4.88-4.90 (m, 1H), 7.02 (d, J ) 8.0 Hz, 1H), 7.46 (d, J ) 8.0 Hz, 2H), 9.89 (s, 1H); MS (EI) m/z 198 (M+); HRMS calcd for C10H11FO3, 198.0692; found, 198.0691. Anal. (C10H11FO3) C, H. 4-(3-Fluoropropoxy)-3-methoxybenzaldehyde (14): 1H NMR (CDCl3) δ 2.22-2.32 (m, 2H), 3.93 (s, 3H), 4.25 (t, J ) 6.5 Hz, 2H), 4.63 (t, J ) 5.5 Hz, 1H), 4.72 (t, J ) 5.5 Hz, 1H), 7.00 (d, J ) 8.0 Hz, 1H), 7.42-7.45 (m, 2H), 9.86 (s, 1H); MS (EI) m/z 212 (M+); HRMS calcd for C11H13FO3, 212.0849; found, 212.0850. 4-(3-Methoxyphenyl)-3-buten-2-one Derivatives (1, 2, and 3). 5-Iodovanillin, 13, or 14 (1.64 mmol) was dissolved in acetone (1 mL) in a pressure tube (ACE glass, 13 × 102 mm). To this solution was carefully added 2.5 M NaOH (1 mL), and the mixture was stirred at room temperature for 24 h. This was then treated with 3 M HCl (5 mL) and then stirred until yellow crystals formed. Products were filtered, washed with water, and then dried. The products, 1 (280.4 mg, 54%), 2 (323.1 mg, 83%), or 3 (280.5 mg, 68%), were obtained as yellow solids after recrystallization from ethanol and water. 4-(4-Hydroxy-3-iodo-5-methoxyphenyl)-3-buten-2-one (1): mp 149.1-151.6 °C; 1H NMR (CD3OD) δ 2.36 (s, 3H), 3.92 (s, 3H), 6.66 (d, J ) 16.0 Hz, 1H), 7.23 (d, J ) 2.0 Hz, 1H), 7.51 (d, J ) 16.0 Hz, 1H), 7.58 (d, J ) 1.5 Hz, 1H); MS (EI) m/z 318 (M+); HRMS calcd for C11H11IO3, 317.9753; found, 317.9751. Anal. (C11H11IO3) C, H. 4-[4-(2-Fluoroethoxy)-3-methoxyphenyl]-3-buten-2-one (2): mp 109.6-113.9 °C; 1H NMR (CDCl3) δ 3.39 (s, 3H), 3.93 (s, 3H), 4.30 (t, J ) 4.5 Hz, 1H), 4.36 (t, J ) 4.5 Hz, 1H), 4.77 (t, J ) 4.5 Hz, 1H), 4.86 (t, J ) 4.5 Hz, 1H), 6.63 (d, J ) 16 Hz, 1H), 6.92 (d, J ) 8.5 Hz, 1H), 7.10-7.14 (m, 2H), 7.47 (d, J ) 16.0 Hz, 1H); MS (EI) m/z 238 (M+); HRMS calcd for C13H15FO3, 238.1005; found, 238.1001. 4-[4-(3-Fluoropropoxy)-3-methoxyphenyl]-3-buten-2-one (3): mp 73.7-75.2 °C; 1H NMR (CDCl3) δ 2.19-2.26 (m, 1H), 2.262.39 (m, 1H), 2.39 (s, 3H), 3.92 (s, 3H), 4.21 (t, J ) 6.0 Hz, 2H), 4.64 (t, J ) 6.0 Hz, 1H), 4.73 (t, J ) 6.0 Hz, 1H), 6.62 (d, J ) 16.0 Hz, 1H), 6.92 (d, J ) 8.5 Hz, 1H), 7.12 (td, J ) 8.5, 2.0 Hz, 2H), 7.48 (d, J ) 16.0 Hz, 1H); MS (EI) m/z 252 (M+); HRMS calcd for C14H17FO3, 252.1162; found, 252.1163. 5-Hydroxy-1-(3-methoxyphenyl)-1,4-hexadien-3-one Derivatives (4, 5, 6, 15, and 16). 2,4-Pentanedione (0.81 mL, 7.92 mmol) and B2O3 (500 mg, 7.18 mmol) were dissolved in ethyl acetate (5 mL), and the solution was stirred at 80 °C for 30 min. To this mixture was added an ethyl acetate solution (10 mL) of the vanillin derivative (13, 14, 5-iodovanillin, vanillin, or 5-bromovanillin (3.6 mmol)) and (n-BuO)3B (0.4 mL, 1.48 mmol). After stirring for 30 min at 80 °C, n-butylamine (0.14 mL, 1.43 mmol) was added dropwise to the mixtures, which were allowed to stir at 100 °C for 1 h. They were then treated with 1 N HCl (10 mL) at 50 °C and stirred at the same temperature for 30 min. Reaction mixtures were extracted with ethyl acetate, washed with water, and then dried over Na2SO4. Flash column chromatography (2:1 hexanes-ethyl acetate), followed by recrystallization from ethanol and water, gave 4 (534.8 mg, 53%), 5 (508.6 mg, 48%), 6 (1.1 g, 85%), 15 (607.2 mg, 72%), or 16 (665.1 mg, 59%) as yellow solids. 1-[4-(2-Fluoroethoxy)-3-methoxyphenyl]-5-hydroxy-1,4-hexadien-3-one (4): mp 105.1-107.1 °C; 1H NMR (CDCl3) δ 2.18 (s, 3H), 3.93 (s, 3H), 4.30 (t, J ) 4.5 Hz, 1H), 4.52 (t, J ) 4.5 Hz, 1H), 4.76-4.77 (m, 1H), 4.85-4.87 (m, 1H), 5.66 (s, 2H), 6.37 (d, J ) 16.5 Hz, 1H), 6.91 (d, J ) 4.0 Hz, 1H), 7.10 (td, J ) 8.3, 2.0 Hz, 1H), 7.56 (d, J ) 15.5 Hz, 1H); MS (EI) m/z 280 (M+); HRMS calcd for C15H17FO4, 280.1111; found, 280.1112. Anal. (C15H17FO4) C, H. 1-[4-(3-Fluoropropoxy)-3-methoxyphenyl]-5-hydroxy-1,4-hexadien-3-one (5): mp 70.0-71.2 °C; 1H NMR (CD3Cl) δ 2.18 (s, 3H), 2.22-2.24 (m, 1H), 2.27-2.30 (m, 1H), 3.92 (s, 1H), 4.21 (t,

Curcumin and Dehydrozingerone DerViatiVes

J ) 6.5 Hz, 2H), 4.64 (t, J ) 6.0 Hz, 1H), 4.73 (t, J ) 6.0 Hz, 1H), 5.66 (s, 2H), 6.36 (d, J ) 15.5 Hz, 1H), 6.91 (d, J ) 8.5 Hz, 1H), 7.06-7.12 (m, 2H), 7.56 (d, J ) 15.5 Hz, 1H); MS (EI) m/z 294 (M+); HRMS calcd for C16H19FO4, 294.1267; found, 294.1266. 5-Hydroxy-1-(4-hydroxy-3-iodo-5-methoxyphenyl)-1,4-hexadien-3-one (6): mp 178.2-180.0 °C; 1H NMR (CDCl3) δ 2.16 (s, 3H), 3.94 (s, 3H), 5.63 (s, 1H), 6.30 (dd, J ) 15.5, 8.0 Hz, 2H), 6.96 (d, J ) 2.0 Hz, 1H), 7.43 (d, J ) 15.5 Hz, 2H), 7.52 (d, J ) 1.5 Hz, 2H); MS (FAB) m/z 361 (M+ + H); HRMS calcd for C13H14IO4, 360.9937; found, 360.9937. Anal. (C13H13IO4) C, H. 5-Hydroxy-1-(4-hydroxy-3-methoxyphenyl)-1,4-hexadien-3one (15): mp 143.5-145.5 °C; 1H NMR (CDCl3) δ 2.15 (s, 3H), 3.94 (s, 3H), 5.63 (s, 1H), 5.84 (s, 1H), 6.32 (d, J ) 16.0 Hz, 1H), 6.93 (d, J ) 15.5 Hz, 1H), 7.09 (d, J ) 10.5 Hz, 2H), 7.53 (d, J ) 16.0 Hz, 1H); MS (EI) m/z 234 (M+); HRMS calcd for C13H14O4, 234.0892; found, 234.0891. 1-(3-Bromo-4-hydroxy-5-methoxyphenyl)-5-hydroxy-1,4-hexadien-3-one (16): mp 158.8-161.4 °C; 1H NMR (CDCl3) δ 1.97 (s, 3H), 3.74 (s, 3H), 6.17 (d, J ) 16.0 Hz, 1H), 6.80 (d, J ) 2.0 Hz, 2H), 7.12 (d, J ) 2.0 Hz, 1H), 7.25 (d, J ) 16.0 Hz, 2H); MS (FAB) m/z 315 (M+ + H, 81Br), 313 (M+ + H, 79Br); HRMS calcd for C13H1481BrO4, 315.0056; found, 315.0048. 5-Hydroxy-1-(4-hydroxy-3-iodo-5-methoxyphenyl)-7-(4-hydroxy-3-methoxyphenyl)-1,4,6-heptatrien-3-one (7) and 1-(3Bromo-4-hydroxy-5-methoxyphenyl)-5-hydroxy-7-(4-hydroxy3-methoxyphenyl)-1,4,6-heptatrien-3-one (17). Vanillin (374 g, 2.46 mmol) and B2O3 (427 mg, 6.13 mmol) were dissolved in ethyl acetate (5 mL) at 80 °C, and to this mixture was added an ethyl acetate solution (5 mL) of 6 or 16 (3.07 mmol) and (n-BuO)3B (1.65 mL, 6.12 mmol). After stirring for 30 min, mixtures were treated with piperidine (121.7 µL, 1.23 mmol) at 80 °C for 30 min and then with 0.4 N HCl (5 mL) at 50 °C for 30 min. Reaction mixtures were extracted with ethyl acetate, washed with water, and then dried over Na2SO4. Flash column chromatography (2:1 hexanes-ethyl acetate) gave 7 (700 mg, 58%) or 17 (549.3 mg, 50%) as orange-colored solids. 5-Hydroxy-1-(4-hydroxy-3-iodo-5-methoxyphenyl)-7-(4-hydroxy-3-methoxyphenyl)-1,4,6-heptatrien-3-one (7): mp 209.9212.4 °C; 1H NMR (CD3OD) δ 3.95 (s, 6H), 5.82 (d, J ) 18.0 Hz, 2H), 6.48 (d, J ) 15.5 Hz, 2H), 6.93 (d, J ) 8.0 Hz, 1H), 7.05 (d, J ) 2.0 Hz, 2H), 7.12 (dd, J ) 8.0, 2.0 Hz, 2H), 7.59 (d, J ) 15.5 Hz, 2H); MS (FAB) m/z 495 (M+ + H); HRMS calcd for C21H20IO6, 495.0305; found, 495.0312. 1-(3-Bromo-4-hydroxy-5-methoxyphenyl)-5-hydroxy-7-(4-hydroxy-3-methoxyphenyl)-1,4,6-heptatrien-3-one (17): mp 207.6211.1 °C; 1H NMR (CD3OD) δ 3.92 (dd, J ) 7.8, 4.0 Hz, 6H), 6.53 (dd, J ) 15.5, 1.5 Hz, 2H), 6.85 (d, J ) 8.0 Hz, 1H), 7.08 (ddd, J ) 20.0, 10.0, 2.0 Hz, 3H), 7.34 (d, J ) 2.0 Hz, 1H), 7.47 (d, J ) 2.0 Hz, 1H), 7.55 (d, J ) 3.0 Hz, 2H); MS (FAB) m/z 449 (M+ + H, 81Br), 447 (M+ + H, 79Br); HRMS calcd for C21H2081BrO6, 449.0426; found, 449.0421. 1-[4-(3-Fluoropropoxy)-3-methoxyphenyl]-5-hydroxy-7-(4-hydroxy-3-methoxyphenyl)-1,4,6-heptatrien-3-one (8) and 1-[4-(3Bromopropoxy)-3-methoxyphenyl]-5-hydroxy-7-(4-hydroxy-3methoxyphenyl)-1,4,6-heptatrien-3-one (18). Compound 15 (337.8 mg, 1.44 mmol) and B2O3 (148 mg, 2.13 mmol) were dissolved in ethyl acetate (5 mL) at 80 °C. To this mixture was added an ethyl acetate solution (5 mL) of 10 or 14 (180 mg, 0.85 mmol) and (nBuO)3B (0.57 mL, 2.11 mmol). After stirring for 30 min, the mixture was treated with piperidine (50 µL, 0.51 mmol) at 80 °C for 30 min, and then with 0.4 N HCl (5 mL) at 50 °C for 30 min. After the reaction mixtures were extracted with ethyl acetate, washed with water, and then dried over Na2SO4, the products were purified by flash column chromatography (2:1 hexanes-ethyl acetate) to give 8 (197.4 mg, 54%) or 18 (155.6 mg, 38%) as yellow solids. In another experiment, 19 (59 mg, 0.10 mmol) was dissolved in THF (5 mL), and to this solution was added n-Bu4NF (1 M in THF, 1 mL, 1.00 mmol). The reaction mixture was refluxed for 4 h, solvent was removed in vacuo, and the product was purified by

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flash column chromatography (2:1 hexanes-ethyl acetate) to afford 8 (10 mg, 23%) as a yellow solid. 1-[4-(3-Fluoropropoxy)-3-methoxyphenyl]-5-hydroxy-7-(4-hydroxy-3-methoxyphenyl)-1,4,6-heptatrien-3-one (8): mp 92.693.2 °C; 1H NMR (CDCl3) δ 2.22-2.33 (m, 2H), 3.91 (s, 3H), 3.94 (s, 3H), 4.20 (t, J ) 6.0 Hz, 2H), 4.62 (t, J ) 5.5 Hz, 1H), 4.72 (t, J ) 5.5 Hz, 1H), 5.81 (s, 1H), 6.47 (dd, J ) 15.5, 7.8 Hz, 2H), 6.93 (dd, J ) 20.0, 9.9 Hz, 2H), 7.02-7.12 (m, 4H), 7.59 (d, J ) 15.5 Hz, 2H); MS (EI) m/z 428 (M+); HRMS calcd for C24H25FO6, 428.1635; found, 428.1638. Anal. (C24H25FO6) C, H. 1-[4-(3-Bromopropoxy)-3-methoxyphenyl]-5-hydroxy-7-(4-hydroxy-3-methoxyphenyl)-1,4,6-heptatrien-3-one (18): 1H NMR (CDCl3) δ 2.36-2.41 (m, 2H), 3.62-3.71 (m, 2H), 3.76 (s, 3H), 3.80 (s, 3H), 4.20 (t, J ) 5.5 Hz, 2H), 5.87 (s, 1H), 5.89 (s, 1H), 6.48 (dd, J ) 15.8, 7.5 Hz, 2H), 6.87-6.94 (m, 2H), 7.02-7.26 (m, 2H), 7.59 (d, J ) 16.0 Hz, 2H); MS (EI) m/z 490 (M+, 81Br), 488 (M+, 79Br); HRMS calcd for C24H2581BrO6, 490.0818; found, 490.0792. 5-Hydroxy-1-(4-hydroxy-3-methoxy-5-tributylstannylphenyl)1,4-hexadien-3-one (20) and 5-Hydroxy-7-(4-hydroxy-3-methoxyphenyl)-1-(4-hydroxy-3-methoxy-5-tributylstannylphenyl)1,4,6-heptatrien-3-one (21). A mixture of the bromo compounds 16 or 17 (0.37 mmol), bis(tributyltin) (1.1 mL, 2.18 mmol), and Pd(Ph3P)4 (84.4 mg, 0.037 mmol) in toluene (10 mL) was stirred at 110 °C overnight. At the end of the reactions, solvent was removed in vacuo and the crude products were purified by flash column chromatography (2:1 hexanes-ethyl acetate) to give 20 (38.7 mg, 20%) as an orange-colored oil or 21 (45 mg, 18%) as an orange-colored solid. 5-Hydroxy-1-(4-hydroxy-3-methoxy-5-tributylstannylphenyl)1,4-hexadien-3-one (20): 1H NMR (CDCl3) δ 0.78-0.96 (m, 9H), 1.03-1.19 (m, 6H), 1.26-1.42 (m, 6H), 1.48-1.69 (m, 6H), 2.17 (s, 3H), 3.94 (s, 3H), 5.66 (s, 1H), 5.98 (s, 1H), 6.33 (d, J ) 17.5 Hz, 1H), 7.01 (d, J ) 2.0 Hz, 1H), 7.13 (d, J ) 2.0 Hz, 1H), 7.58 (d, J ) 16.0 Hz, 1H); MS (FAB) m/z 547 (M+ + Na); HRMS calcd for C25H40O4SnNa, 547.1851; found, 547.1860. 5-Hydroxy-7-(4-hydroxy-3-methoxyphenyl)-1-(4-hydroxy-3methoxy-5-tributylstannylphenyl)-1,4,6-heptatrien-3-one (21): mp 95.1-97.5 °C; 1H NMR (CDCl3) δ 0.85-0.98 (m, 9H), 1.121.47 (m, 18H), 3.94 (d, J ) 8.0, 6H), 5.91 (s, 2H), 6.46 (d, J ) 16.0 Hz, 2H), 6.93 (d, J ) 9.5 Hz, 2H), 7.00-7.14 (m, 2H), 7.527.59 (m, 3H), 7.71 (dd, J ) 8.8, 5.0 Hz, 1H); MS (FAB) m/z 659 (M+ + H); HRMS calcd for C33H47O26Sn, 659.2395; found, 659.2407. 5-Hydroxy-1-(4-hydroxy-3-[125I]iodo-5-methoxyphenyl)-1,4hexadien-3-one([125I]6) and 5-Hydroxy-1-(4-hydroxy-3-[125I]iodo5-methoxyphenyl)-7-(4-hydroxy-3-methoxyphenyl)-1,4,6-heptatrien-3-one ([125I]7). To 100 µL of the tributylstannyl precursor 20 or 21 (1 mg/mL ethanol) were added 1 N HCl (100 µL) and Na[125I]I (185 MBq). Reactions were initiated by adding 100 µL of H2O2 (3% w/v) and stirring at room temperature for 10 min. After adding saturated NaHSO3 (300 µL, aq), radiolabeled products were extracted using ethyl acetate and passed through an anhydrous Na2SO4 plug. Solvent was then removed under a gentle stream of N2, and reaction mixtures were purified by HPLC using a semipreparative column eluted with a 60:40 mixture of CH3CN and trifluoroacetic acid (0.1%, aq) at a flow rate of 3.0 mL/min. The desired fraction of [125I]6 was eluted between 19 and 20 min, whereas [125I]7 was eluted between 18 and 19 min using a 55:45 mixture of CH3CN and trifluoroacetic acid (0.1%, aq) at a flow rate of 4.0 mL/min. Specific activities were determined by comparing UV peak areas of the desired radioactive peak and the UV peak areas of different concentrations of unlabeled standard 6 or 7 on HPLC. Aliquots of radioligand ([125I]6 or [125I]7) were coinjected with unlabeled ligand into the HPLC system to confirm identities. 1-[4-(3-[18F]Fluoropropoxy)-3-methoxyphenyl]-5-hydroxy-7(4-hydroxy-3-methoxyphenyl)-1,4,6-heptatrien-3-one ([18F]8). [18F]Fluoride (1110 MBq) was placed in a Vacutainer containing n-Bu4NOH (3.2 µL, 4.88 µmol). Three azeotropic distillations were then performed using 100-200 µL aliquots of CH3CN at 90 °C

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(oil bath) under a gentle stream of N2. The resulting n-Bu4N[18F]F was then dissolved in CH3CN (200 µL) and transferred to a reaction vial containing precursor 12 (2 mg, 5.49 µmol). The reaction mixture was then stirred at 105 °C for 10 min to give [18F]14. The resulting mixture was added to an ethyl acetate solution (200 µL) of 15 (2.5 mg, 10.7 µmol) and B2O3 (2 mg, 28.7 µmol), and to this solution were added (n-BuO)3B (10 µL, 37.1 µmol) in ethyl acetate (200 µL) and piperidine (1 µL, 10.1 µmol). This reaction mixture was stirred at 105 °C for 15 min and then treated with 0.4 N HCl (300 µL) at 90 °C for 5 min. The mixture was cooled, diluted with water (2 mL), and extracted with ethyl acetate (2 mL). The organic layer was washed with water and passed through a 3-cm Na2SO4 plug, and solvent was removed under a stream of N2 at 50 °C (water bath). The reaction mixture was then purified by HPLC using a semipreparative column eluted with a 60:40 mixture of CH3CN and trifluoroacetic acid (0.1%, aq) at a flow rate of 3.0 mL/min. The desired product eluted between 23 and 24 min. [18F]8 was identified and its effective specific activity was determined as described above. In another experiment, n-Bu4N[18F]F was dissolved in THF (300 µL) and transferred to a reaction vial containing precursor 19 (0.9 mg, 1.45 µmol). The reaction mixture was then stirred at 95 °C for 20 min and passed through a short plug filled with silica gel and Na2SO4 using a 9:1 mixture of CH2Cl2 and CH3OH. The remainder of the procedure was as described above. Binding Assays Using Aβ(1-40) Aggregates. Aβ(1-40) peptide (0.5 mg) was dissolved in sodium phosphate buffer (10 mM, 1 mL) containing 1 mM EDTA (pH 7.4) and stirred gently at 37 °C for 40-42 h. Binding studies were carried out using the method described in the literature.14,18 For saturation binding studies, Aβ(1-40) aggregates (final concentration 20-50 nM) were added to mixtures containing 125I-labeled standard, 125I-IMSB or 125I-IMPY (50 µL, 0.01-8 nM in 40% ethanol). The final concentration of ethanol in a total volume of 1 mL of solution was 10%. Nonspecific binding was determined in the presence of 10 µM CG or thioflavin-T (50 µL in 40% ethanol). For inhibition studies, a total volume of 1 mL containing the ligands (50 µL, 10-5-10-10 M in 40% ethanol) and 0.1 nM of radiolabeled standard (50 µL in 40% ethanol) was used. Reaction mixtures were incubated with shaking at room temperature for 3 h, and bound radioactivity was collected on Whatman GF/B filters using a cell harvester (Inotech Biosystems, Dottikon, Switzerland) and washed twice using 3 mL of 10% ethanol. The filters containing radioactivity were then counted using a gamma counter at a counting efficiency of 83%. Data were analyzed using software Prism, and Kd and Ki values were calculated. Partition Coefficient Measurement. Radioligands, [125I]6, [125I]7, or [18F]8, collected from HPLC, were concentrated under a gentle stream of N2 and redissolved in 10% ethanol-saline. The radioligands were respectively added to premixed suspensions containing 600 µL octanol and 600 µL water and then vortexed vigorously for 3 min and centrifuged at 2000 rpm for 5 min. Two layers separated out, and 100 µL aliquots of the octanol and aqueous layers were removed and counted. Samples from the octanol and aqueous layers were repartitioned until consistent values were obtained. The experiments were carried out in triplicate. Log Po/w was expressed as the logarithm of the ratio of the counts per minute from octanol versus that of water.39 Metabolism Studies. Mice (ICR, male 25-30 g) were injected with [18F]8 (14.8 MBq/mouse) via a tail vein and sacrificed at 2, 30, and 60 min post-iv-injection. At the designated time points, samples of blood and whole brain were collected, homogenized in 1 mL of chilled absolute ethanol, and centrifuged at 3000 rpm for 3 min. Supernatants were analyzed by radio-TLC using a 20:1 mixture of CH2Cl2-CH3OH and a 1:1:0.01 mixture of CH2Cl2CH3OH-triethylamine. Biodistribution in Normal Mice. ICR mice (male, 25-30 g, four mice per time point) were injected with [125I]6 (0.74 MBq) or [18F]8 (1.85 MBq) in 0.2 mL of 10% ethanol-saline via a tail vein. The mice were sacrificed at the indicated times (2, 30, 60, and 120 min). Samples of blood, heart, lung, liver, spleen, kidney, muscle,

Ryu et al.

brain, and thyroid (or bone) were removed, weighed, and counted. Data are expressed as the percent injected dose per gram of tissue (% ID/g). In another experiment, mice were injected with a mixture of radioligand [18F]8 and piperine (2 mg/kg) in 0.2 mL of 10% ethanol-saline via a tail vein. The remainder of the procedure was as described above.

Acknowledgment. We thank Ms. Hyun Jung Jeon for performing the biodistribution studies and Mr. Dong Hyun Kim and Iljung Lee for their technical assistance. The tributylstannyl precursor of [125I]IMSB was a generous gift from the University of Pennsylvania (PA). This study was supported by a grant of the Korea Health 21 R&D Project, Ministry of Health & Welfare, Republic of Korea (A050329). References (1) Ginsberg, S. D.; Schmidt, M. L.; Crino, P. B.; Eberwine, J. H.; Lee, V. M. Y.; Trojanowski, J. Q. Molecular Pathology of Alzheimer’s Disease and Related Disorders. In Cerebral Cortex: NeurodegeneratiVe and Age-Related Changes in Structure and Function of Cerebral Cortex; Peters, A., Morrison, J. H., Eds.; Kluwer Academic/ Plenum: New York, 1999; pp 603-654. (2) Lee, V. M.; Trojanowski, J. Q. Neurodegenerative Tauopathies: Human Disease and Transgenic Mouse Models. Neuron 1999, 24, 507-510. (3) Selkoe, D. J. The Origins of Alzheimer Disease: A is for Amyloid. JAMA 2000, 283, 1615-1617. (4) Mathis, C. A.; Wang, Y.; Klunk, W. E. Imaging β-Amyloid Plaques and Neurofibrillary Tangles in the Aging Human Brain. Curr. Pharm. Design 2004, 10, 1469-1492. (5) Tubis, M.; Blahd, W. H.; Nordyke, R. A. The Preparation and Use of Radioiodinated Congo Red in Detecting Amyloidosis. J. Am. Pharm. Assoc. 1960, 49, 422-425. (6) Mathis, C. A.; Mahmood, K.; Debnath, M. L.; Klunk, W. E. Synthesis of a Lipophilic, Radioiodinated Ligand with High Affinity to Amyloid Protein in Alzheimer’s Disease Brain Tissue. J. Labelled Compd. Radiopharm. 1997, 40, 94-95. (7) Dezutter, N. A.; De Groot, T. J.; Busson, R. H.; Janssen, G. A.; Verbruggen, A. M. Preparation of 99mTc-N2S2 Conjugates of Chrysamine G, Potential Probes for the beta-Amyloid Protein of Alzheimer’s Disease. J. Labelled Compd. Radiopharm. 1999, 42, 309-324. (8) Dezutter, N. A.; Dom, R. J.; De Groot, T. J.; Bormans, G. M.; Verbruggen, A. M. 99mTc-MAMA-Chrysamine G, a Probe for β-Amyloid Protein of Alzheimer’s Disease. Eur. J. Nucl. Med. 1999, 26, 1392-1399. (9) Han, H.; Cho, C. G.; Lansbury, P. T. Technetium Complexes for the Quantitation of Brain Amyloid. J. Am. Chem. Soc. 1996, 118, 4506-4507. (10) Zhen, W.; Han, H.; Anguiano, M.; Lemere, C. A.; Cho, C. G.; Lansbury, P. T. Synthesis and Amyloid Binding Properties of Rhenium Complexes: Preliminary Progress Toward a Reagent for SPECT Imaging of Alzheimer’s disease Brain. J. Med. Chem. 1999, 42, 2805-2815. (11) Levin, V. A. Relationship of Octanol/Water Partition Coefficient and Molecular Weight to Rat Brain Capillary Permeability. J. Med. Chem. 1980, 23, 682-684. (12) Abraham M. H. The Factors that Influence Permeation Across the Blood-Brain Barrior. Eur. J. Med. Chem. 2004, 39, 235-240. (13) Styren, S. D.; Hamilton, R. L.; Styren, G. C.; Klunk, W. E. X-34, a Fluorescent Derivatie of Congo Red: a Novel Histochemical Stain for Alzheimer’s Disease Pathology. J. Histochem. Cytochem. 2000, 48, 1223-1232. (14) Skovronsky, D. M.; Zhang, B.; Kung, M. P.; Kung, H. F.; Trojanowski, J. Q.; Lee, V. M. Y. In Vivo Detection of Amyloid Plaques in a Mouse Model of Alzheimer’s Disease. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 7609-7614. (15) Zhuang, Z. P.; Kung, M. P.; Hou, C.; Skovronsky, D. M.; Gur, T. L.; Trojanowski, J. Q.; Lee, V. M. Y.; Kung, H. F. Radioiodinated Styrylbenzenes and Thioflavins as Probes for Amyloid Aggregates. J. Med. Chem. 2001, 44, 1905-1914. (16) Agdeppa, E. D.; Kepe, V.; Liu, J.; Flores-Torres, S.; Satyamurthy, N.; Petric, A.; Cole, G. M.; Small, G. W.; Huang, S. C.; Barrio, J. R. Binding Characteristics of Radiofluorinated 6-Dialkylamino-2Naphthaylethylidene Derivatives as Positron Emission Tomography Imaging Probes of β-Amyloid Plaques in Alzheimer’s Disease. J. Neurosci. 2001, 21, RC189.

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